Geographical Distribution. 179 



no one has yet given more than a very general account 

 of the causes of the present distribution of plants and 

 animals even in a small area like Britain ; yet one step 

 of progress is at least secure it has been recognized 

 that the result is due to the co-operation of many 

 factors, and that any solution which does not recognize 

 all the factors that are known is bound to be fallacious. 

 The chief factors, which have been alluded to in the 

 previous paragraphs, are: (i) the constitution of the 

 organism; (2) the physical conditions of the region; 

 (3) the position of the original headquarters of the 

 stock; (4) the means of dispersal both active and pas- 

 sive; (5) the historical changes of the earth's crust and 

 climate; and (6) the bionomic conditions which involve 

 a struggle for existence. 



Although life is almost cosmopolitan, most of its 

 forms have become adapted to particular conditions, 

 and are more or less restricted to these. It The Great 

 is thus possible to make a much wider and Faunas and 

 more fundamental grouping than that into Floras - 

 geographical realms; we may inquire into the distinc- 

 tive population of the littoral, pelagial, abyssal, fluvial, 

 and terrestrial areas (Lebensbezirke), and discuss their 

 possible historical relations to one another. To this line 

 of inquiry much attention has been directed of recent 

 years, and although the problem is a fine instance of 

 "reach exceeding grasp ", many valuable results have 

 already been gained. 



By littoral we mean the area from high-tide mark to a 

 depth of about 100 fathoms, where the plateau surround- 

 ing the continents ends. It is the smallest 

 of the five chief areas in actual surface, but 

 probably the richest in life. It includes a few flowering 

 plants, e.g. Zostera, that can endure submergence, the 

 great majority of the sea-weeds, and representatives of 

 all the chief classes of animals except amphibians. From 

 the time of Edward Forbes onwards much ingenuity has 

 been expended in dividing the littoral area into zones in 

 reference to the Algae, the animals, or the nature of the 

 substratum. It is an area of great physical variety, 

 subject to continual vicissitudes, and much influenced 



